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260

WHITE METAL

TIN-FOIL

Tin-foil, properly so called, should consist of thin sheets of pure tin
and is hence analysed as described for tin. Often, however, it is formed
of lead and tin, or of lead, antimony and tin, and in some cases is tinned'
to obtain the shining white colour of tin. With these last two types of
tin-foil, the most important determination is that of the tin, this being
effected as for the tin alone in lead-tin alloys (see Alloys of Lead and Tin,
p. 258). A complete analysis may be carried out by the method indicated
for the analysis of lead-tin alloys, when these are the only constituent metals,
or by that given for white metal in the case of an alloy of lead, antimony
and tin.

If the foil is tinned externally, the coating must be removed in order
that the quantity of tin actually present in the alloy may be determined.
If the foil is moderately thick, the surface layer may be removed mechani-
cally from the two faces and the quantitative determination made on the
sample thus prepared. If, however, the foil is too thin and weak to be
scraped, the surface tin is removed by means of sodium hydroxide solution
and hydrogen peroxide,, which, at the proper concentration, easily attacks
and dissolves the layer of tin, whereas the tin in the alloy is acted on either
not at all or but slightly. The conditions to be observed are as follows *:

From 5 to 6 grams of the foil are freed from grease by treatment with
ether, cut into squares of about I cm. side and heated in a 200 c.c. flask
with 20 c.c. of 10% sodium hydroxide to 40, 6-8 drops of 30% (100 volume)
hydrogen peroxide being then added with continual shaking. The surface
tin passes rapidly into solution and as soon as the lead-grey colour assumed
by the pieces of metal indicates that this process is complete, the alkaline
solution is decanted off and the residue washed repeatedly with water,
alcohol and ether successively and dried at 60-70. The foil being thus
freed from the surface tin, the quantitative determination of the components
of the alloy is carried out by the methods indicated above.

WHITE  METAL

This includes many types of alloys with a basis of tin-antimony, lead-
antimony or lead-tin-antimony, together with small quantities of copper
and sometimes zinc and iron (rarely mercury and arsenic), used as antifriction
metals,' type metal, metal for fittings, etc. The composition, and
therefore the analytical methods to be used, vary with the purpose of
the metal. Three groups may be distinguished :

(1)  Alloys with a basis of tin (70-85%) and antimony (10-15%), with
a little copper (5-10%) and lead (0-5-2%), and perhaps zinc and iron.

(2)  Alloys with a basis of lead (60-80%), antimony (10-15%) and ^
(10-25%), with a little copper (0-5-1%) and perhaps zinc, iron and arsenic.

(3)  Alloys with a basis of lead (75-90%) and antimony (10-25%), with
a small quantity of tin and copper (0-5-2%).

1 Eelasio : " Determination of Tin in Foil of Lead, Tin and Antimony Tinned Ex-
teirnally" (Ann. Labor. CMm. Gabelle, VI, p. 231).readily fusible